He may have been killed along with other members of the DHARMA Initiative in the Purge, or he may have left the island sometime between 1977 and The Purge.

Trivia

Mitchell's jumpsuit

Rudy, who portrayed Mitchell, also portrayed another DHARMA member who wore a grey jumpsuit. The member was presented at "the vote" to determine if Sayid should be executed, helped other members to put out the fire caused by young Ben ("He's Our You"), and presented during Horace's speech after the fire had been put out ("Whatever Happened, Happened"). Although the name that labeled on his jumpsuit was hard to determine, it can be assumed that it wasn't Mitchell, and the two were different characters.